What is a canonical link element?

    1. A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the “canonical”, or “preferred”, version of a web page as part of search engine optimization.
      1. Example: <link rel=”canonical” href=”http://email.biz/” />
      2. While the canonical link element has its benefits, Matt Cutts, who is the head of Google’s webspam team, has claimed that the search engine prefers the use of 301 redirects. Cutts claims the preference for redirects is because Google’s spiders can choose to ignore a canonical link element if they feel it is more beneficial to do so
    2. Used for SEO that will describe the ‘proper’ link that should be used
    3. For example ‘Show All’ on a category listing page as opposed to the individual pages

There are a few canonical extensions out there in the Magento eco-system but I have used the FME Canonical URLs  extension with some success.